Displacement sensors of this type are used to detect displacement distances in machines or motor vehicles. A sensor using a solenoid plunger is a commonly used device for making these kind of measurements. With this kind of device, a magnetically conductive part, such as the solenoid plunger, is pushed into a coil. The solenoid plunger is connected to the part of a machine or motor vehicle whose displacement distance is to be measured. The inductance of the coil changes as a function of the displacement of the inserted solenoid plunger. Preferably, the inserted plunger is made of iron. This inductance change is evaluated by a suitable electronic system and is converted into a corresponding displacement distance. In order to increase the inductance and to insulate the sensor from outside interference, the coil is generally enclosed in a magnetically conductive housing. The magnetically conductive housing may be in the shape of a pot. In order to maintain the magnetic conductivity of the circuit at a high level, the air gap between the solenoid plunger and the housing is kept as small as possible.
The above-mentioned electronic system can use various techniques to evaluate the displacement-dependent inductance of the coil. For example, it is customary to subject the inductance of the coil to a voltage surge and then evaluate the resulting increasing current over time. The resulting increasing current is evaluated up to a set limit.
A displacement sensor which operates as described above is known in the art. For example, DE-A1 38 07 015 (U.S. Pat. No. 4,859,942) discloses a displacement sensor of this kind. An electronic evaluation circuit is also known from this patent document, which uses the variable inductivity of the coil to determine a displacement value.
A basic problem with all the known solenoid plunger displacement sensors is their sensitivity to outside interferences. This may cause the sensor to respond differently when it is installed in a machine or a motor vehicle, than when it is not installed.
Three different types of interferences may occur. First, static or changing magnetic interference fields may occur at the installation site. Second, iron parts may be installed at the installation site in the vicinity of the sensor which may influence the leakage field of the sensor and thereby may lower its measuring precision. Third, electrically conductive parts constructed of a metal other than iron that are located in the vicinity of the sensor may influence the measuring process due to their eddy current effects. In addition, this will also lower the precision of the displacement measurement.